In recent years, enhancement of fuel efficiency of automobiles has become an important issue from the viewpoint of global environment protection. Consequently, there is an active movement to reduce the thickness of vehicle body components through increases in strength of vehicle body materials, and thereby reduce the weight of vehicle body itself.
In general, however, strengthening of steel sheets leads to deterioration in formability, causing the problem of cracking during forming. It is thus not simple to reduce the thickness of steel sheets. Therefore, it is desirable to develop materials with increased strength and good formability. In addition to good formability, steel sheets with a tensile strength (TS) of 980 MPa or more are required to have, in particular, enhanced impact energy absorption properties. To enhance impact energy absorption properties, it is effective to increase yield ratio (YR). The reason is that a higher yield ratio enables the steel sheet to absorb impact energy more effectively with less deformation.
For example, JPS61157625A (PTL 1) proposes a high-strength steel sheet with extremely high ductility having a tensile strength of 1000 MPa or higher and a total elongation (EL) of 30% or more, utilizing deformation induced transformation of retained austenite.
In addition, JPH1259120A (PTL 2) proposes a high-strength steel sheet with well-balanced strength and ductility that is obtained from high-Mn steel through heat treatment in a ferrite-austenite dual phase region.
Moreover, JP2003138345A (PTL 3) proposes a high-strength steel sheet with improved local ductility that is obtained from high-Mn steel through hot rolling to have a microstructure containing bainite and martensite after subjection to the hot rolling, followed by annealing and tempering to cause fine retained austenite, and subsequently tempered bainite or tempered martensite in the microstructure.